The present invention relates to an impeller for an axial flow fan comprising a hub supporting the blades, the hub having bores in its peripheral surface, whereby each bore has a first radially inner section with larger diameter than a second radially outer section, each blade having a substantially circular foot section allowing adjustment of the blade pitch, each blade foot section being mounted in a corresponding bore by means of connecting plates engaging with a shoulder between the two bore sections.
In conventional axial flow fans of this type the blade foot section is clamped against six connecting plates, each of which constitutes a segment of a circular disc, the segments being arranged in the bore in spaced relationship in order to permit mutual displacement by insertion and removal of the plates from the bore section. At the centre of the arrangement of the segments the borders of the segments jointly form a circular hole, in which a retaining ring is fitted in a groove at insertion of the segments in order to keep these in place while mounting the blade foot section and especially during subsequent rotation of the foot section in the bore for adjustment of the blade pitch. The connecting plates are pressed against the shoulder in the bore by means of a spring placed in the central hole. However, by the known fans, it is a disadvantage that the blade foot section in certain cases may have a tendency to lock in its corresponding bore in the hub, thereby impeding adjustment of the pitch.
The object of the invention is to provide an impeller of the type described in the introduction, whereby the above mentioned problem is remedied, and whereby at the same time a solid connection between the blade and the hub is obtained.
In view of this object, the impeller according to the invention is characterized in that each blade foot section is mounted by means of two connecting plates which in the first bore section are spaced apart and face each other along substantially the entire diameter of the bore.
By arranging two larger connecting plates in this way, instead of using six smaller in the conventional way, the connecting plates are far less inclined to tilt by rotation in the bore and thereby impede adjustment of the blade in relation to the hub. Furthermore, the larger plates provide sufficient space for the provision of more than one bolt in respect of each plate for clamping the blade foot section against the plate. This further reduces the tendency of the plates to tilt in the bore, thereby also facilitating the adjustment of the blade pitch. Moreover, the contact surface between the connecting plates and the shoulder in the bore may in this way be constituted by two relatively long continuous areas, as opposed to six discretely arranged small areas in the prior art impellers. The more uniform structure of the total contact area between the connecting plates and the shoulder provides less frictional force by rotation of the blade foot section. Furthermore, a much more uniform stress distribution may be obtained in the hub around the respective bores, as well as in the foot section of the blade, against which the connecting plates are clamped, this resulting in a considerably stronger connection between blade and hub.
In an advantageous embodiment very simple to manufacture the connecting plates have sections, the periphery of which extend substantially along a circle having a diameter larger than that of the second bore section in the hub, and the largest dimension of each plate is shorter than the diameter of said bore section. In this manner, a uniform, continuous contact surface is obtained between the connecting plate and the shoulder in the corresponding bore, thereby providing a further improved stress distribution, especially in the hub material surrounding the bore. In addition, the two connecting plates necessary for mounting one blade can be machined by turning one circular disc, cutting it through diametrically, thereby removing sufficient material for provision of the space necessary between the two plates in their mounted position, and eventually, if necessary, cutting off the extreme tips to allow the plates to enter through the second bore section.
In a further advantageous embodiment, a radial clearance is provided between the periphery of the connecting plates and the first bore section. In this manner, adjustment of the blade pitch may be further facilitated in that tilting of the connecting plates during rotation of the blade cannot cause engagement between the periphery of the connecting plates and the wall material of the first bore section.
A blade shaft projecting from the blade foot section may be clamped against an elastic member located in the bore under the connecting plates. By providing the elastic member under the connecting plates, the member does not take up any space between the plates, and no cutout or other reduction of the plate material, which would reduce its strength, is required.
The elastic member may have the form of one or more disc springs located in a hole in the bottom of the bore in the hub, which hole may have substantially the same diameter as the outer diameter of the springs. In this manner, the elastic member is kept in place when mounting the blade, possibly further by a retaining ring mounted in the hole.
The present invention also relates to a blade connecting means for mounting an impeller blade on a hub for an axial flow fan, the connecting means comprising connecting plates adapted to be clamped against a foot section of the blade and engage in a corresponding bore in the hub with a shoulder between a first radially inner bore section with larger diameter than a second radially outer bore section. According to the invention, the blade connecting means is characterized in that there is provided two connecting plates which are adapted to be mounted in the first bore section spaced apart and facing each other along substantially the entire diameter of the bore.
The invention further relates to a blade for an axial flow fan comprising the above described blade connecting means.
The invention also relates to a method of mounting a blade on a hub for an axial flow fan, whereby a blade foot section is clamped against connecting plates located in a corresponding bore in the peripheral surface of the hub, and the connecting plates are caused to engage with a shoulder between a first radially inner bore section with larger diameter than a second radially outer bore section. According to the invention, the method is characterized by clamping the blade foot section against two connecting plates which are spaced apart and face each other along substantially the entire diameter of the bore.
The connecting plates may be positioned in such a manner that the periphery of sections of the plates extend substantially along a circle having a diameter larger than that of the second bore section in the hub, and each of the plates utilized may have a maximum dimension that is shorter than the diameter of said bore section.
The connecting plates may be positioned in such a manner that a radial clearance is provided between the periphery of the connecting plates and the first bore section.